Influence of the substrate on order and image contrast for physisorbed, self-assembled molecular monolayers: STM studies of functionalized hydrocarbons on graphite and MoS2

Scanning tunneling microscopy (STM) has been used to investigate the influe nce of different substrates on monolayer ordering and molecular image contr ast for singly and doubly substituted long-chain hydrocarbons. Self-assembl y at the phenyloctane-graphite and phenyloctane-MoS2 interfaces can be mild ly or strongly affected by the underlying surface. In the most extreme case s, changes in the ordering of two-dimensional adlayers involve both rotatio n and translation of the molecules. Differences in the electronic character istics of the underlying surface also have important implications for the S TM image contrast exhibited by many molecules. Careful inspection of functi onal group contrast on both substrates provides useful information about th e relative importance of geometric/topological versus electronic coupling f actors in the STM tunneling mechanism for physisorbed molecules.